"I haven't been able to bring myself to write most people back to thank them for their condolences," the 31-year-old told People magazine on Monday. "I am heartbroken. I am still in disbelief.

"We laid him to rest on Saturday. He is in the place he was always certain he wanted to spend eternity."

Crystal, who was married to Hefner for five years, also praised her late partner, calling him an "American hero" and a "pioneer".

"A kind and humble soul who opened up his life and home to the world," she said.

She said she was "eternally grateful to have been by his side" as Hefner "taught me kindness" and gave her direction in her life.

"He changed my life, he saved my life. He made me feel loved every single day. He was a beacon to the world, a force unlike anything else. There never has and never will be another Hugh M Hefner."

Hefner was laid to rest next to Marilyn Monroe at Westwood Village Memorial Park Cemetery in Los Angeles. He is survived by his daughter Christie Hefner, 64, and sons David Hefner, 62, Marston Hefner, 27, and Cooper Hefner, 26.

"I join the world in mourning. I thank you for all of your condolences. To the people leaving sentiments at the front gate, we see you and grieve with you," Crystal added.